1. Field of the Invention
This invention relates to plasma gas chambers. More particularly, this invention relates plasma gas chambers employing one or more paired electrodes for exciting a gas to a plasma state.
2. Description of the Prior Art
"Plasma" has been defined as matter in an "elevated state". Physical properties of the plasma state are somewhat similar to those in the gaseous state. The flow patterns and characteristics become extremely aggressive in the plasma state, where the gases are essentially in a nascent form. Plasma or these "active species" are created in a region between a pair of RF (radio frequency) electrodes, and are then directed toward the target surface.
Conventional plasma gas chambers include a cylindrical or rectilinear chamber with a front-loading door and either a barrel or planar electrode configuration. The objects to be exposed to the plasma gas are inserted between the electrodes either vertically or horizontally. The gases, generally oxygen (O.sub.2) and a carbon tetrafluoride (CF.sub.4), are introduced into the chamber to flow between the electrode pair to be excited to a plasma state by the electrical field created between the paired electrodes. The objects are enveloped by the plasma gas and processed according to the desired methods. U.S. Pat. Nos. 4,148,705, 4,230,553, 3,661,747, 4,123,663, 4,148,705, 3,276,106, 3,598,710, 4,287,851, 4,289,598, 4,282,077, 4,012,307, 4,264,393, 4,227,321, 4,285,800, 4,307,283, 4,151,034, 4,033,287, 4,158,589, 4,192,706, 4,198,261, 3,410,774, 3,867,216, 3,705,055, 3,930,913, 4,178,877, 4,026,742, and 4,123,663, British Pat. No. 1,522,059, and French Pat. No. 2,308,200, the disclosure of each of which are incorporated by reference herein, illustrate many configurations of electrode pairs in the treatment of objects such as semiconductor substrates and multi-layer printed circuit boards.
Recently, improved plasma gas chambers have been developed by me comprise a rectilinear main body portion having frustro-pyramidal diffusion chambers on opposing ends thereof. Porous or apertured electrode pairs are positioned transversely in the rectilinear chamber. The gas is introduced into one of the frustro-pyramidal chambers and pulsated back and forth through the apertured electrode pairs from one diffusion chamber to the other. The objects, having been placed between the adjoining electrode pairs, are consequently treated by the plasma gas. Extremely uniform results are obtained because the gas is uniformly excited to a plasma state as it passes through the primary electrical field created by the power and ground electrodes of each electrode pair. The plasma gas thus created is significantly maintained as it flows to the adjoining pair because it is exposed to a secondary electric field created by the power or ground electrode of one electrode pair and the ground or power electrode of the succeeding electrode pair, respectively. U.S. Pat. Nos. 4,425,210 and 4,328,081, the disclosure of each of which is hereby incorporated by reference herein, set forth a more detailed description of this unique plasma gas chamber employing paired electrodes.
My plasma gas chambers have achieved great commercial success in the removal of smear in the holes of multi-layer printed circuit boards. Still more recently, I have used my novel plasma gas chamber, or variations thereof, in the sputtering of a metal onto the inside surface of the holes in printed circuit boards. These plated-through-hole apparati and methods are disclosed in my co-pending applications, Ser. No. 383,311, filed May 28, 1982 and Ser. No. 438,168, filed Apr. 8, 1983, entitled "Plated-Through-Hole Apparatus and Method", the disclosure of each of which is incorporated by reference herein.
In all of my chambers, the electrode pairs consisted of two sheets of aluminum or copper in which was drilled or punched a multiplicity of uniformly dispersed holes. These two sheets of metal were then positioned parallel to one another by means of ceramic stand-offs. One of the sheets was connected to the RF generator to act as a power electrode and the other was connected to the ground of the system to act as the ground electrode.
These electrode pairs have been reasonably sufficient to accomplish the objectives of my previous inventions. However, there has always been a need to increase the rate of plasma desmearing and sputtering to increase the efficiency and throughput of the chambers. Unfortunately, there has always been competing parameters which limit such efficiency and throughput. Specifically, the rate of desmearing and sputtering can be increased by positioning the electrode pairs closer to one another within the chamber. However, this decreases the number of printed circuit boards which may be inserted therebetween, thereby decreasing the throughput. Conversely, while spacing electrode pairs further apart allows more printed circuit boards to be treated, the secondary electrical field created between the adjoining electrode pairs is significantly decreased when operating at the same power level, thereby decreasing the ability to maintain the gas in a plasma state. Increased power levels are usually undesirable since they significantly increase the temperature of the plasma gas within the chamber and, consequently, resulting in overheating of the printed circuit boards. Even without increased power levels, the electrode pairs progressively warped and deformed until eventually they would be discarded and new ones installed.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the plasma chamber art.
Another object of this invention is to provide a paired electrode for plasma chambers which function to increase the efficiency and thoroughput of the chamber during operation.
Another object of this invention is to provide a paired electrode for plasma chambers which has a unique construction which can be used in conjunction with existing plasma chambers and which can be easily manufactured at relatively low cost as compared to existing paired electrodes.
Another object of this invention is to provide a paired electrode for plasma chambers which is significantly more durable in construction than that of existing paired electrodes thereby increasing the life of the paired electrodes.
Another object of this invention is to provide a paired electrode for plasma chambers having a construction which precludes warpage or deforming during use throughout its life.
Another object of this invention is to provide a paired electrode for plasma chambers which can be manufactured from stock materials thereby eliminating all difficult machining operations.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.